To complete a well, often one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun may be lowered into the wellbore and fired to create openings in a casing and to extend perforation tunnels into the surrounding formation zones.
Pressure in the wellbore can also be manipulated in relation to the formation zones to achieve removal of debris from perforation tunnels or to achieve enhanced fluid flow from the formation zones. The pressure manipulation includes creating a transient underbalance condition (when the wellbore pressure is lower than the formation pore pressure) prior or subsequent to detonation of a detonation cord or shaped charges of limited energy. Pressure manipulation also includes creating a transient overbalance condition (when the wellbore pressure is higher than the formation pore pressure) prior or subsequent to detonation or explosion of shaped charges of a perforating gun or a propellant. Creation of an underbalance condition can be accomplished in a number of different ways, such as by use of a low pressure chamber that is opened to create the transient underbalance condition, use of empty space in a perforating gun or tube to draw pressure into the gun right after firing, and use of other techniques. The underbalance condition results in a suction force that extracts debris out of the perforation tunnels, allowing formation fluid to flow more efficiently into the wellbore or injection fluids to flow more efficiently into the formation zones. Creation of an overbalance condition can be accomplished by use of a propellant (which when detonated causes high pressure gas buildup), use of a pressurized chamber, or use of other techniques. The overbalance condition can cause pressure to increase to a sufficiently high level to fracture the formation zones. Fracturing allows for better communication of formation fluids into the wellbore or better injection of fluids into the formation zones.
Before perforation and before subsequent manipulation of wellbore pressure, one or more packers or plugs can be positioned between the inside of the wellbore and the outside of the perforating gun or underbalance or overbalance device to isolate the interval over which the detonation, explosion, or actuation takes place to achieve a quicker and amplified response for the perforation or for the underbalance or overbalance condition.